J Mater Sci (2007) 42:729–746

DOI 10.1007/s10853-006-0401-4

REVIEW

Mechanism of geopolymerization and factors influencing

its development: a review
Divya Khale Æ Rubina Chaudhary

Received: 22 September 2005 / Accepted: 2 May 2006 / Published online: 20 January 2007
Ó Springer Science+Business Media, LLC 2007

Abstract Geopolymerization is a developing field of Introduction

research for utilizing solid waste and by-products. It
provides a mature and cost-effective solution to many Industrialization leads to the generation and release of
problems where hazardous residue has to be treated undesirable pollutants into the environment. In order
and stored under critical environmental conditions. to keep pace with the rapid industrialization there is a
Geopolymer involves the silicates and aluminates of necessity to select such process, which would cause
by-products to undergo process of geopolymerization. minimum pollution in environment.
It is environmentally friendly and need moderate In recent years, there is an increasing awareness on
energy to produce. This review presents the work the quantity and diversity of hazardous solid waste
carried out on the chemical reaction, the source generation and its impact on human health. Increasing
materials, and the factor affecting geopolymerization. concern about the environmental consequences of
Literature demonstrates that certain mix compositions waste disposal has led to investigation of new utiliza-
and reaction conditions such as Al2O3/SiO2, alkali tion avenues [1].
concentration, curing temperature with curing time, The greatest problem faced by industries, as far as
water/solid ratio and pH significantly influences the waste disposal is concerned is the safe and effective
formation and properties of a geopolymer. It is utilized disposal of its effluent, sludge and by-products such as
to manufacture precast structures and non-structural large quantities of fly ash that are produced during the
elements, concrete pavements, concrete products and combustion of coal used for electricity generation. It is
immobilization of toxic metal bearing waste that are estimated that by the year 2010, the amount of the fly
resistant to heat and aggressive environment. Geo- ash produced will be about 780 million tones annually
polymers gain 70% of the final strength in first 3–4 h of [2]. Most of this ash is disposed in landfills at suitable
curing. sites [1, 3]. Landfilling is not a desirable option because
it not only causes huge financial burden to the
foundries, but also makes them liable for future
environmental costs and problems associated with
landfilling regulations [4]. The increasing load of toxic
metals in the landfill potentially increases the threat to
ground water contamination.
Increasing economic factors also dictate that indus-
try should look forward to recycling and reuse of waste
D. Khale
R. Chaudhary (&) material as a better option to landfilling and discarding.
Hazardous Waste Management Laboratory, School of Need exists for a technology that can easily and
Energy And Environmental Studies, Devi Ahilya
cheaply handle large quantities of waste materials and
University, Takshila Campus, Khandwa Road,
Indore 452001 MP, India by-products containing heavy metals as an alternative
e-mail: rubina_chaudhary@yahoo.com to OPC (ordinary Portland cement).

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730 J Mater Sci (2007) 42:729–746

Disposal of hazardous waste must meet at least two [5]. The inorganic polymeric material can be considered
conditions [5] as an amorphous equivalent of geological feldspars, but
synthesized in a manner similar to thermosetting organic
(1) Safe chemical encapsulation i.e. control their
polymers. For this reason, these materials are termed as
release into ground water and seepage water.
‘‘geopolymers’’[11].
(2) Structural stability with respect to adverse envi-
It offers attractive option for simple industrial
ronmental condition.
applications where large volume of waste materials
Production of one ton of Portland cement requires needs to be stabilized [5]. It is named because of the
about 2.8 ton raw materials, including fuel and other similarities with the organic condensation polymers as
materials and generates 5 to 10 % of dusts. Altogether far as their hydrothermal synthesis conditions are
6000–14000 m3 dust-containing air-streams are gener- concerned [8]. Study of the literature and patents
ated per ton cement manufacture, which contain demonstrated, that before 1978, the idea of using this
between 0.7 to 800 g/m3 of dust and accounts for about mineral chemistry for the development of a mineral
one ton of green house gas CO2 released to the polymer had been totally neglected. As a function of
atmosphere as a result of de-carbonation of lime in the chemical composition of initial materials, the alkaline
kiln during manufacturing of cement (Eq. 1) [2, 6, 7]. cements are classified into two groups.
(i) Binders synthesized from materials rich in calcium
5 CaCO3 þ 2 SiO2 ! 3 CaO
SiO2
such as blast furnace slag that produces calcium
þ 2 CaO
SiO2 þ 5 CO2 ð1Þ silicate hydrate (CSH) gel when activated with
alkaline solution.
A technology was therefore sought as an alternative (ii) Materials synthesized with raw materials low in
to the afore-mentioned standards and, further more, the calcium and rich in SiO2 and Al2O3 such as
cost figures must not be intolerable [5]. To overcome metakaolin. These materials when activated with
these problems, geopolymers emerged as a possible alkaline solution, formation of an amorphous
solution for using the by-products and could be utilized material (alkaline aluminosilicate) that develops
to manufacture precasts structure and non-structural high mechanical strength at early ages after a soft
elements, concrete pavements, concrete products and thermal curing [12].
immobilization of toxic waste that are resistant to heat These materials differ substantially from ordinary
and aggressive environment [8]. The objective of this Portland cement, as they use totally different reac-
review is to study the work carried out on the develop- tion pathway in order to attain structural integrity.
ment of geopolymers, including the chemical reaction, Pozzolanic cement depends on the presence of
the role and effect of the source materials, and the calcium-silicate hydrate for matrix formation and
factors affecting mix compositions, such as curing strength where as geopolymers utilize the polycon-
temperature, curing time, Al2O3/SiO2 ratio in the mix, densation of silica and alumina precursors (fly ash,
alkali concentration, pH and water/solid ratio. kaolin, metakaolin) and a high alkali content to
attain structural strength [13].

Geopolymerization
Chemistry of geopolymer
Geopolymerization is a geosynthesis (reaction that
chemically integrates minerals) that involves naturally Geopolymerization is based on chemistry of alkali
occurring silico-aluminates [5]. Any pozzolanic com- activated inorganic binders, which were accidentally
pound or source of silica and alumina, that is readily discovered by Purdon [14]. He studied the sodium
dissolved in the alkaline solution, acts as a source of hydroxide on a variety of minerals and glasses con-
geopolymer precursor species and thus lends itself to taining silicon and/or aluminum and summarized it in
geopolymerization [9]. The alkali component as an two steps;(1) liberation of silica, alumina and lime and
activator is a compound from the element of first group (2) formation of hydrated calcium silicates, aluminates
in the periodic table, so such material is also called as as well as regeneration of caustic solution. Author
alkali activated aluminosilicate binders or alkali acti- proposed that the hardening mechanism of alkali
vated cementitious material [10]. Silicon and aluminum activated alumino silicate binder involves dissolution
atoms react to form molecules that are chemically and of Si or Al in the presence of sodium hydroxide, and
structurally comparable to those building natural rocks precipitation of calcium silicate or aluminum hydrate

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J Mater Sci (2007) 42:729–746 731

with the generation of sodium hydroxide. Similarly, M2O/Al2O3, 0.8 to 1.6 [16, 31, 32]. The geopolymeric
Glukhovsky identified both CSH and calcium and alumino-silicate have been grouped in three families
alumino-silicate hydrate as solidification product on depending on the atomic ratio Si/Al that may be 1,2,or
the alkali activation of slag binders and concluded that 3 [32].
clay mineral reacts during alkali treatment to form
aluminosilicate hydrate. Finally, Davitovits [15] devel-
oped a kind of mineral polymer material with 3-D Reaction Involved in Geopolymerization
cross-linked polysialate chain, which resulted from the
hydroxylation and polycondensation reaction of natu- NaOH/KOH (-)
(Si2O5,Al2O2)n + 3nH2O n(OH)3-Si-O-Al-(OH)3 (2)
ral minerals such as clay, slag, fly ash and pozzolan on
alkaline activation below 160 °C (Fig. 1). This inor-
ganic polymer was first named polysialate in 1976 and (-) (-)
NaOH/KOH
later coined as ‘‘Geopolymer’’[15]. In 1980 the setting n(OH)3-Si-O-Al-(OH)3 (Na,K) –Si-O-Al-O-)n + 3nH2O (3)

reaction of alkali activated slag cement was explained O O

Orthosialate (Na,K)-Polysialate
[14, 16]. Recently, Deventer [9, 13, 14, 16–29] has
contributed towards the development and applications
of geopolymers. (-)
NaOH / KOH
Mechanism of geopolymers involves the polycon- (Si2O5,Al2O2)n + nSiO2 + 4n H2O n(OH)3-Si-O-Al-(OH)3 (4)

densation reaction of geopolymeric precursors i.e. (OH)2

alumino-silicate oxide with alkali polysiliates yielding
polymeric Si–O–Al bond [8, 16, 30, 31].
(-) NaOH / KOH (-)
n(OH)3-Si-O-Al-(OH)3 (K,Na)-(Si -O-Al-O-Si-O-)n + 4n H2O (5)
Mn ½ðSi  O2 Þz  Al  On
wH2 O
(OH)2 O O O

Ortho(sialate-siloxoxo) (Na,K)-Polysialate-siloxo
where M is the alkaline element, z is 1,2, or 3 and n is
the degree of polycondensation [30]. Geopolymers consist of aluminum and silica tetra-
Davitovits has suggested that certain synthesis limits hedrally interlinked alternately by sharing all the
existed for the formation of strong products; satisfac- oxygen atoms. A polymeric structure of Al–O–Si
tory compositions lay in the range M2O/SiO2, 0.2 to formed constitutes the main building blocks of
0.48; SiO2/Al2O3, 3.3 to 4.5; H2O/M2O, 10–25; and geopolymeric structure. Alkali metal salts and/or

Fig. 1 Computer molecular

graphics of polymeric Mn–
(–Si–O–Al–O–)n poly(sialate) + NaOH/KOH

Poly(sialate) Na-Poly(sialate) framework K-Poly(sialate) framework

(PS)

SiO 4 AlO 4

+ NaOH/KOH

(Na,K)- Poly(sialate-siloxo) (Na,K)-Poly(sialate-siloxo) KPoly(sialatesiloxo)

(PSS) Framework Framework

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732 J Mater Sci (2007) 42:729–746

hydroxide are necessary for the dissolution of silica and

alumina as well as for the catalysis of the condensation
reaction. The gel phase is thought to be highly reactive
and produced by co-polymerization of individual alu-
mina and silica from their source, dissolved by the
alkali metal. Some cations must be present to keep the
structure neutrality (since aluminum is four fold). Na,
K, Ca and other metallic cations maintain this neutral-
ity. It is still not clear whether these ions simply play a
charge-balancing role or are actively bonded into the
matrix. The mechanism of immobilization is expected
to be the combination of chemical and physical
interaction. Cation is either bonded into the matrix Fig. 2 Crystallization temperature for geopolymers and zeolites
via Al–O or Si–O bond or present in the framework
cavities to maintain electrical charge balance. A
physically encapsulated cation should be substituted which are chemically less reactive. Zeolites usually
by another cation if its surrounding allows the diffusion crystallize from dilute aqueous solution, where precur-
process to occur [14]. Amorphous to semi-crystalline sor species have mobility as well as enough time to
three dimensional alumino-silicate structures are of the undergo proper orientation and alignment before
poly(sialate) type (Si–O–Al–O–), the poly(sialate-sil- bonding into a crystal structure.
oxo) type (Si–O–Al–O–Si–O–) , the poly(sialate-disil-
oxo) type(Si–O–Al–O–Si–O–Si–O–) [8].
The coordination number of the silica and alumina Role of materials in geopolymer
in the source material is of great importance. Some of
the water and NaOH are expelled out during harden- Starting material plays an important role in the
ing of the gel phase. It is believed that the alkali metal formation of geopolymer. Several studies have been
hydroxide acts as a catalyst and leach out from the conducted in order to develop various methods to
hardened alkali activated binder in more or less the improve the durability of the geopolymeric cement and
same amount as that was added during synthesis. Ions concrete. Wide range of materials is presently being
such as metal cations that are incorporated into the used for geopolymerization. These include various
matrix in one way or another influence the final pozzolanic, supplementary cementitious materials,
structure stability and therefore leaching characteris- chemicals and mineral additives [36]. Materials rich
tics [14]. in Si (like fly ash, slag and rice husk) and materials rich
Utilization of pozzolans results in the added tech- in Al (clays like kaolin, bentonites) are the primary
nical advantages like reduction in temperature rise, requirement to undergo geopolymerization. Some
improvement in durability and strength enhancement. other materials can also be utilized as a reactive filler
Although in some cases strength develops slowly [33]. material or a setting additive like ordinary Portland
Geopolymers are very similar to zeolites and are cement, kiln dust etc. which helps in the development
formed in similar manner as zeolites do [16]. There of good mechanical properties. Some of the important
exists a difference between zeolite formation and materials used for the geopolymerization are described
geopolymerization as related to the composition of in the following sub-section.
the initial reaction mixtures. Zeolites usually form in
closed hydrothermal systems but geopolymers do not. Fly ash
Geopolymers are amorphous to semi-crystalline,
where as zeolites are usually crystalline in nature. Fly ash is one of the important source materials for
When fly ash in geopolymers are mixed with the geopolymer. It is available world wide yet its utilization
alkaline dissolution; a vitreous component is quickly is still limited. Fly ash consists of finely divided ashes
dissolved. In such a situation there is not sufficient time produced by burning pulverized coal in power stations.
and space for the gel to grow into a well-crystallized The chemical composition depends on the mineral
structure resulting in a microcrystalline, amorphous or composition of the coal gangue (the inorganic part of
semi-amorphous structure [34, 35] (Fig. 2). Aluminum the coal). Silica usually varies from 40 to 60 % and
source react with calcium hydroxide via a pozzolanic alumina from 20 to 30%. The iron content varies quite
reaction to produce CSH and calcium alumino silicate, widely. Alkalis are present in an appreciable amount

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J Mater Sci (2007) 42:729–746 733

and potassium prevails over sodium [37]. When CaO is incorporating admixture, such as 0.01% ZnO to the mix
greater than 20% then it can be categorized as [37]. Some times cement is also used as a calcium source,
cementitious material. When CaO varies between for the formation of strong geopolymer, which contrib-
10% and 20% categorized as cementitious and pozzo- utes tricalcium silicate (C3S) and di-calcium silicate (C2S)
lanic material. A pozzolanic material requires calcium with the small amount of tricalcium aluminate (C3A) and
hydroxide (CH) in order to form strength-imparting calcium aluminoferrite (C4AF) [40, 41].
products (pozzolanic activity). Usually the CaO con-
tent in these material is not enough to react with all the Kiln dust
quantity of the pozzolanic compounds, and exhibit
pozzolanic activity (pozzolanic and cementitious mate- Lime kiln dust and cement kiln dust act as absorbents
rial). It is used with Portland cement, which yields CH and bulking agent. Lime kiln dust, also acts as a
on hydration [38, 39]. For geopolymerization high neutralizing agent in acidic condition. Kiln dusts are
alkaline solutions are used to induce the silicon and effective due to their calcium oxide content. This gives
aluminum atom in the source material to dissolve and them high alkalinity and ability to remove free water
form geopolymeric paste [2]. by the hydration of CaO to Ca(OH)2 . The actual
Fernandez-Jimenez and Palomo [35] reported that setting reaction of kiln dusts are pozzolanic and
finesses of the fly ashes plays an important role in the resembles to those of portland cement in many ways.
development of the mechanical strength of the mate- Kiln dust contains silica and silicates from their natural
rial obtained after activation. They reported that, when rock genesis, with cement kiln dust generally having
the particle fraction sized higher than 45 lm. is much higher silica content than the lime kiln dust [40].
removed, mechanical strength increased remarkably,
reaching 70 MPa in one day. van Jaarsveld et al. [18] Alkali activators
reported that the surface charge on the fly ash particle
affects the initial setting properties of a geopolymeric Strong alkalis are required to activate the silicon and
mix. This is because the mechanism of dissolution and aluminum present in the fly ash and setting additives,
subsequent geopolymerization involves the initial that allows transforming glassy structure partially or
transportation of hydroxyl ions to the surface of the totally into a very compacted composite [2, 42]. The
fly ash particles. This is followed by hydrolysis, thereby common activators are NaOH, Na2SO4, waterglass,
forming aluminate and silicate species. Subsequent Na2CO3, K2CO3, KOH, K2SO4 or a little amount of
polymerization of these monomers forms oligomers of cement clinker [10]. Sodium silicate has been used for
varying geometries that causes the formation of the more than a century for the production of commercial
geopolymeric gel phase. products such as special cements, coatings, molded
articles and catalysts. Some times silica fume is used as
Burned clay and OPC an alternative to the sodium silicate, which normally
forms part of the reactant solution [43]. The soluble
The burning of certain clays (kaolin, bentonites, silicate is mixed with fly ash, cement, lime, slag or other
montmorillonite etc) and oil shales produces ashes, source of multivalent metal ions that promotes the
that harden when mixed with lime and water. Clay gelation and precipitation of silicates. More the NaOH
mineral, such as Kaolin, gains a distinct pozzolanic get in contact with the reactive solid material the more
activity when burned at temperature between 600 °C silicate and aluminate monomers are released. Below a
and 900 °C. These artificial pozzolanas are mostly ratio of solution to solid of about 50 the concentration
composed of silica and alumina. The loss of combined of the released monomers reaches saturation [44].
water due to the thermal treatment causes the crystal- In Fig. 3, bars represents release of silicate and
line network of the clay mineral to be destroyed, while aluminate monomers in 10% NaOH solution and
silica and alumina remains in a messy, unstable symbols (triangle and circle) represents concentration
amorphous state, that reacts with CH [33]. The of released monomers at different L/S ratios.
pozzolanic activity depends on the clay mineral content During the pozzolanic reaction alkali cation might
and thermal treatment conditions. Hardening results get incorporated in the hydration product. It is believed
from the presence of cementitious compounds such as that the alkalis are incorporated within the interlayer of
C2S and CS. the CSH phase mostly by neutralizing SiOH group. The
The highest strength obtained with mixes made of amount of alkali hydroxide incorporated increases with
burned kaolin (metakaolin) was 27 MPa. The mechanical decrease in the CaO/SiO2 mole ratio in the mix [45].
properties of the calcined clay can be improved by Soluble silicate reduces the leachability of toxic metal

123
734 J Mater Sci (2007) 42:729–746

Fig. 3 Influence of NaOH

concentration on the release
of silicate and aluminate
monomers from aluminum
source

ions by forming low-soluble metal oxide/silicates and by Factor affecting unconfined compressive strength
encapsulation of metal ions in the silicate-or silicate-gel
matrix [40]. Curing temperature

The curing temperature is an important factor in the

Superplasticizers setting of the concrete [47]. Pozzolanic reactions are
accelerated by temperature increase (Table 1) [30, 52,
Addition of the superplasticizers improves the work- 53]. At ambient temperature, the reaction of fly ash is
ability of the geopolymeric paste by improving the extremely slow [54]. Initial curing at elevated temper-
plastic and hardening properties, leading to higher ature catalyzes formation in appropriate system chem-
compressive strength. Compressive strength value istry [55]. Brooks [47] reported that, for Type I cement
depends on the pore structure of the hardened super- and fly ash concrete, setting time was decreased by a
plastizer paste, which consists mainly of micropores factor of six when pozzolanic reactions were acceler-
leading to a denser structure [46]. ated by temperature increase from 6 °C to 80 °C.
Increase in the curing temperature in range of 30 °C to
90 °C increased the compressive strength [2, 52].
Curing at 70°C improved the strength compared to
Mechanical property of geopolymer: unconfined the curing at 30 °C for the same period of time. Curing
compressive strength at higher temperature, for more than a couple of hours,
possibly affects the development of the compressive
In order to produce a geopolymer with a high strength (Fig. 4 and 5) [4, 38].
compressive strength, source materials with a high Investigations of Kirschner et al. [56] demonstrated
reactivity are required [23]. The interaction among the that, processing at ambient temperature was unfeasible
source materials and between the source material and due to a delayed beginning of setting. However this
the gel phase should also be considered. Kaolinite, with could be avoided by thermal treatment. He also
a comparatively lower reactivity allows sufficient time reported that curing at 75 °C for 4 h completed a
for such interactions to occur and ultimately increases major part of geopolymerization process and resulted
the extent of geopolymerization. Not only a significant in satisfactory properties of the material. No secondary
improvement in the compressive strength, but also a treatment is required thereafter.
substantial reduction in reaction time can be achieved Similarly, Swanepoel and Strydom [52] investigated
when a calcined source material (Fly ash) is added to utilization of fly ash and kaolinite clay in the geopoly-
the geopolymerization of non-calcined materials meric material. The compressive strength after 7 and
(Kaolinite). A higher strength geopolymer is associated 28 days was highest (6 and 7 MPa respectively) for the
with a more desirable internal microstructure [17]. sample heated at 60 °C for 48 h. Increase in temperature

123
 Table 1 Factors affecting on the strength of geopolymers and other binders
S.No Al2O3 / M2O/ Alkali Curing in Curing Main Setting Heavy Metal W/ Hydroxide Silicate Al2O3/ UCS Others Ref
SiO2 SiO2 Metal Oven (d) Component Additives (mass %) S (mass %, (mass M2O (MPa)
(mass%) M) %,
Temp Time M)
(°C) (h)

1 0.57 – K 70 24 7 FA Kaolinite – 0.31 [13]

(15%)
2 – – Na 50 24 1 FA Builder’s Cu (0.5%) 0.2 4% 3.5% – 75 – [14]
waste (15%)
J Mater Sci (2007) 42:729–746

3 0.57 1.14 K 30 24 7 FA Kaolinite Cu (0.1%) 0.2 – – – 51.4 – [18]

(15%)
4 0.6 – Na 23 7 7 FA Kaolinite (14 Pb, Cu (0.5%) 0.33 2.5% 2.5% – 32.7 – [19]
%)
5 0.57 – K 30 24 7 FA Kaolinite Cu (0.1%) 0.2 5% – – 51.4 – [20]
(15%)
6 0.61 – Na 45 24 7 FA Cement (12%) Pb, Cu (0.5%) 0.32 – – – 50 – [21]
7 0.62 – Na 45 24 7 FA Kaolinite 0.3 3.8% 3.8% – 60 Zr(3%) [22]
(10%)
8 – 5.134 K 40 7 7 FA FA:Kao:Albite – 0.3 – – – 45.8 [23]
(4:2:1)ratio
9 – – K 20 – 10 – Metakaolin – 0.35 8 M 2.5 – 24.5 Ultrasonication [24]
(60 g) for
5 min + 160 g
sand
10 – 0.81 – 85 24 1 FA – – 0.3 – – – 68.7 [42]
11 – – – 60 48 28 – – 0.2 5% 5% – 8 – [52]
12 – – Na 38 56 FA CKD (50%) – 0.52 2% – – 27 – [53]
13 – – Na 25 5 28 FA Slag (50%) – 0.35 10 M – – 50 – [54]
14 0.30 0.25 Na 65 1.25 NA – Kaolinite – – NA – – 54 – [54]
(NA)
15 – Na 75 30 30 FA 0.3 8M NA 45 2 h pre curing at [58]
room
temperature
Crushed granite
and fine sand
16 – – Na 21 – 1 FA – B (15,000 ppm) 0.6 10 M – – 48 [62]
17 0.32 0.95 K 60 3 1 FA Metakaolin – – 10 M – 0.33 79 – [63]
18 0.26 0.095–0.12 Na 90 24 7 FA 0.17 70 Na2OSiO2/ [72]
NaOH = 2.5 by
mass
19 – – Na 85 24 28 FA – – 0.3 8 M – – 33.77 [77]
20 – – Na 85 24 28 FA – Pb (3.125%) 0.3 8 M – – 25.05 [73]
21 none none none none none 28 OPC none none 0.43 15.2 [48]
(pure)
22 none none none none none 28 OPC None Cr (620 mg/l),Ni 0.43 none none none 1.0 [48]
(329 mg/l),Pb
(359 mg/l),As
(824 ,g/l), Mo
(87 mg/l) ,Zn
735

123
(237 mg/l)
736 J Mater Sci (2007) 42:729–746

from 45 °C to 65 °C, the viscosity of the mix increased

[48]

[48]

[49]

[50]

[51]
Ref
five times however increasing temperature from 65 °C to
85 °C increased the viscosity 10 times [57]. Viscosity
indirectly indicates the geopolymer’s compressive
strength.
Palomo et al. [42] observed mechanical strength of
Hydroxide Silicate Al2O3/ UCS Others

60 MPa, after curing fly ash at 85°C for 5 h, and stated

that temperature is especially important for 2 h to 5 h
(mass %, (mass M2O (MPa)

of curing (Fig. 4). Bakharev [58] stated that heat is

2.1

1.5

7.0

1.4

3.9
beneficial for the strength development (strength
compared to1 month of curing at elevated temperature
none

none

none

none

none
can develop in only 24 h). Wang et al. [53] reported
that CKD-Fly ash when cured at 24 °C, give lower
strength (6.9 and 13.8 MPa at 28 and 56 days respec-
none

none

none

none

none
M)
%,

tively). But the strength doubled at elevated temper-

ature. This was not same for the OPC. On curing at
elevated temperature, OPC exhibit expansion behavior
0.25 none

0.18 none

0.56 none

0.35 none

0.44 none

and leads to cracking, resulting in loss of strength, a

M)

decreased service life or other durability problem [59].

W/

K-PS and K-PSS products gave higher mechanical

S

strength of about 32.5 MPa at 25 °C and 45 °C. The

Pb(359), As(824),

Pb(359), As(824),
Mo(87), Zn(237)

Mo(87), Zn(237)

Cu (1400 mg/l),

mechanical strength obtained with added NaOH at this

Cr (620), Ni(329),
Cr (620) Ni(329),

Cr (400 mg/l),
Cd (5.1 mg/l)
Zn (38.5 mg/l),

temperature, can be used for manufacturing of pre-

Cd (240 mg/l),
Heavy Metal

Pb(0.70%)

formed building blocks [60].

(mass %)

It can be concluded that curing at elevated temper-

ature is effective (in the rage of 30 °C to 90 °C) and has
more significant contribution to geopolymeric reactions.
FA (75%)

FA (60%)
(5.25%)
Component Additives

Curing time
(mass%)

(20%)

(15%)
Setting

GGBS

PFA

Slag

Table 1 reveals that prolonged curing time improve the

polymerization process resulting in higher compressive
strength. However, increase in strength for curing
periods beyond 48 h. was not very significant [2, 42, 52,
Curing Main

OPC

OPC

OPC

OPC

OPC

61]. Geopolymers developed compressive strength of

45 MPa in just 24 h. [62]. Puertas et al. [54] observed
that the compressive strength for one day was higher,
(d)

none 28

none 28

none 28

none 28
none 7

when the curing was carried out at 65 °C, and at rest of

Temp Time

the age, paste cured at 25 °C developed higher com-

(°C) (h)
Curing in

pressive strength than those treated at 65 °C. The

Oven

none

none

none

none

none

increasing temperature favors the dissolution of reactive

species mainly, that of the slag and fly ash, in the same
Alkali

degree. Compressive strength decreased on curing at

Metal

none

none

none

none

none

higher temperature for longer period of time, as

prolonged curing at elevated temperature, breaks the
granular structure of geopolymer mixture. This results in
M2O/

none

none

none

none

none

dehydration and excessive shrinkage due to contraction

SiO2
Table 1 continued

of gel, without transforming to a more semi-crystalline

form. Crystalline part of geopolymer does not get
S.No Al2O3 /

affected by longer curing time. This indicates that, the

none

none

none

none

none
SiO2

change responsible for the difference in the strength

originates within the amorphous phase of the structure
23

24

25

26

27

[13].

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J Mater Sci (2007) 42:729–746 737

Fig. 4 Variation of 40
compressive strength 30°C
withtime at different curing 35 50°C
70°C
temperature

Compressive strength(MPa)
30

25

20

15

10

0
0 10 20 30 40 50 60
Time (hrs)

Fig. 5 Effect of curing 90

temperature on compressive
Compressive strength at 7 days (MPa)

80
strength
70

60

50

40

30

20

10

0
0 20 40 60 80 100 120
Curing time (hrs)

Silicate and hydroxide ratio higher than 0.8, the low temperature reaction yields an
amorphous alumino-silicate or ‘‘inorganic polymer
The ratio of sodium silicate to sodium hydroxide glass’’ whereas for smaller values the sodium silicates
plays an important role in the compressive strength are partially crystalline [64]. In the production of
development (Table 1). M2O/SiO2 ratio shows a inorganic polymer, the amount of OH- ion in the
positive effect on compressive strength. By the increase alkaline solution contributes towards the dissolution
in concentration of alkali M2O (M represents Na/K/ step of Si4+ and Al3+ from fly ash, whilst the Na+ ion
metallic ions) or decrease in added silicate SiO2, contributes to the crystallization of zeolites P.
increase in compressive strength is expected [2, 23,
36]. This is because excess sodium silicate hinders Alkali concentration
water evaporation and structure formation [63]. The
matrix activated with potassium silicate/KOH obtained Alkali concentration is the most significant factor for
the greatest compressive strength while sodium sili- geopolymerization (Table 1) [54]. The solubility of
cate/NaOH activated matrixes were generally weaker aluminosilicate increased with increasing hydroxide
followed by potassium silicate/NaOH. Since K+ is more ion concentration [65]. Higher concentration of NaOH
basic it allows higher rate of solubilized polymeric yielded high compressive strength [2]. 10 N KOH
ionization and dissolution and leading to dense poly- showed the highest strength of 60 MPa, but the
condensation reaction that provides greater overall strength decreased on increasing the KOH concentra-
network formation and an increase in the compressive tion from 10 N to 15 N, probably due to excess K+ ions
strength of the matrix. Large size K may favor a in the framework [63].
greater degree of polycondensation [21, 60]. If SiO2/ K2O/Na2O content plays an important role, with
M2O in the sodium silicate solution is equal to or increasing alkali concentration, setting time increases

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738 J Mater Sci (2007) 42:729–746

and the strength and fire resistance characteristics can mechanical strength [68]. Higher Na proportion in the
also be improved [63]. KOH leached substantially solution would favor the formation of the CSH gel
more Si and Al as compared to NaOH [18, 24]. [17]. As the amount of sodium hydroxide increase in
Addition of K2O was found to benefit the compressive the system, less calcium would be available to react
strength and also to reduce the occurrence of cracking with the silicate and aluminate and calcium will
[23]. High NaOH addition accelerated chemical disso- precipitate out as calcium hydroxide with the forma-
lution but depressed ettringite and CH formation tion of CSH gel. The CSH gel in such system will have
during the binder hydration [53]. Reduction in the significantly lower Ca/Si ratio than the CSH formed
CH content resulted in superior strength and durability from hydration of ordinary Portland cement [27]. Yip
performance [66]. An excess of OH– concentration in et al. [27] in their study also reported that sodium
the system decreased the strength of the system [42]. concentration in the geopolymeric gel was much
Higher the alkalinity of the hydration water, slower the higher than that in calcium rich area (CSH gel). This
rate of the hydration [61]. Wang et al. [53] performed indicates that, sodium added had a more predominant
study using fly ash and CKD with 2% and 5% NaOH. structurally determining role in the alumino-silicate gel
They reported that, addition of 5% NaOH tends to than in the case of CSH gel. This further suggests that
increase the strength of the binder at early age (below sodium played a charge-balancing role in the geopoly-
7 days) but the strength decreased at later age, may be meric gel while it was not required in formation of
that the excessive NaOH that resulted in undesirable CSH gel.
morphology and non-uniformity of hydration products
in the pastes, thereby reducing the binder strength. pH
Kaps and Buchwald [44] reported that measurable
strength could not be built below NaOH content of The most significant factor controlling the compressive
15% whereas above 25% there was no improvement in strength is pH. The setting time of cement decreased as
the strength. Querol et al. [67] in the study of alkali the pH of the activating solution increased [69]. At
activated ferro-aluminous fly ash reported that no lower pH values the geopolymeric mix remained
reaction occurred on activating fly ash using distilled viscous and behaves like cement while at higher pH,
water even at high activation temperature of 150 °C the mix attained a more fluid gel composition, which
because of the low content of free calcium oxide in the was less viscous and is more workable [21]. Strength at
original fly ash and low total basicity (final pH = 6.8) pH 14 was 50 times larger than those at pH 12(less than
which was inadequate for the formation of zeolitic 10 MPa at pH 12, 50 MPa at pH 14) of geopolymeric
phases. matrix utilizing cement as setting additive. Higher
Delay in the polymer formation occurred as activa- solubility of monomers was expected by KOH than
tor concentration was increased. The ionic species NaOH because of higher alkalinity (Fig. 6). With
concentration also increased, limiting the ion’s mobil- increasing pH there was a predominance of smaller
ity and delaying the formation of coagulated structures chain oligomers and monomeric silicate available to
[12]. When the alkali hydroxide concentration was react with soluble aluminum. Further with increase in
increased from 5 M to 10 M, an amorphous alkaline pH soluble aluminum increases and reacts with calcium
alumino silicate (geopolymer) was formed as the available for reaction. [21, 22, 29]. Lower pH-value of
dominant product, with a small amount of CSH gel the solution leads to lower monomer concentration.
[27, 68]. If enough calcium was added to the a Figure 6 reveals the pH-value of the single alkaline
geopolymeric system, some form of CSH gel would solution, varying in concentration and kind of alkali
be obtained. However it is still unclear whether ions [44].
calcium participates in geopolymerization in a similar From the above observations it is clear that pH
way to sodium or potassium [27]. Similarly, when range 13–14 is most suitable for the formation of the
activation of metakaolin was carried out with highly geopolymers with better mechanical strength.
concentrated alkaline solution in the presence of
calcium hydroxide, the main reaction product was
aluminosilicate. Additionally the formation of CSH gel Silicate and aluminum ratio
was also observed as a secondary product [12]. It was
assumed that the CSH gel fills the voids and pores A high soluble silicate dosage is necessary for synthe-
within the geopolymeric binder. This helps to bridge sizing alumino-silicate gel that provides good interpar-
the gaps between the different hydrated phases and ticle bonding and physical strength of geopolymers
unreacted particles, thereby resulting in the increased (Table 1) [25]. High reactive silica content involved the

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J Mater Sci (2007) 42:729–746 739

formation of high amount of alkali alumino-silicate gel Table 2 The compressive

Matrix UCS (MPa)
and consequently a high mechanical strength was strength of fly ash based geo-
polymers synthesized using Kaolin 32.7
developed in the resulting material [35]. different Al source
Geopolymers with SiO2/Al2O3 in the range of 3.16– MK 26.8
K-Feldspar 13.9
3.46 had better UCS which decreased with increasing Fly ash 7.7
ratio upto 3.86 [63]. Results of Bakharev [58] indicated
that utilization of long precuring before heat treatment
allowed to narrow the range of Si/Al ratios in the after 28 days. More than 15% replacement induced a
alumino-silicate gel (Si/Al = 1–4 for 2 h precuring) decreased binding and compressive strength [24]. As
compared to (Si/Al = 1.8–3.6 for 24 h precuring). He the secondary Al–Si source, metakaolin enhanced the
also reported that an increase in curing temperature geopolymerization of fly ash. This was explained by
caused reduction in Si/Al ratios (Si/Al = 1.8–3.6 for the fact that metakaolin tends to dissolve a large
75 °C compared to Si/Al = 1.6–2.8 for 95 °C). Higher extent than fly ash [24]. Due to the fineness of
sodium silicate concentration was found to be benefi- metakaolin grain, the consistency of mix design
cial to the geopolymerization of the Metakaolin/sand decreases. Five percent metakaolin induces a decrease
mixture [24]. of 14.6% in consistency while 15% induces decrease 34
% consistency. Hence, the optimum reactivity seems
Aluminum source to be between 10% and 15% regarding the low
workability, best mechanical performance and inhibi-
Geopolymer containing clays (Kaolin and metakaolin) tion effect on the chloride diffusion and sulfate attack.
were found to be the strongest under compressive No positive or negative effect of kaolin was observed;
strength testing while fly ash alone lacked considerable it may be due to the mineralogical characteristics of
strength alone [19] (Table 2). Strength increased with the material and lack of thermal treatment of the
the addition of Zirconia (3%)suggested that zirconia alumino-silicate in the base material [70]. Matrix with
could efficiently be consolidated into the matrix and calcium source (cement) showed the highest compres-
above 5% caused a reduction in the strength of matrix sive strength 50 MPa as compared to metakaolin and
[22]. A small amount of clay content gets fully digested slag [21].
to take part in geopolymerization reaction where as at
larger addition, clay becomes partially reactive filler Liquid/solid ratio
and serves to weaken the structure [13]. Replacement
by metakaolin induced a decrease in the mechanical Strength decreases as the ratio of water-to-geopolymer
strength during the first day but an equal resistance solid by mass increases. This trend is analogous to

Fig. 6 Influence of the

concentration and kind of the
alkaline solution on the pH-
value

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740 J Mater Sci (2007) 42:729–746

water-to-cement ratio in the compressive strength in the kaolinite samples has a great influence on the
OPC. Although chemical processes involved in the nature of the intermediate phases, but less in that of
formation of binders of both are entirely different the final zeolite synthesized with the same Si/Al
(Table 1) [2, 18, 30, 36, 71, 72]. The minimum water to ratio. During the synthesis of the zeolites from
cement ratio is approximately 0.4 by weight for Portland alumino-silicates, the first formed zeolites are often
cement [40], whereas the fresh geopolymeric material is metastable thermodynamically and are subsequently
readily workable even at low liquid/solid ratio [73] replaced by other more stable one [75]. The double-
(Fig. 7). Presence of excess amount of water is an layered alumino-silicate material can be transformed
important factor inducing crystallization in M2O– to the sol-gel system only after complete dehydroxy-
Al2O3–SiO2–H2O and M2O–CaO–Al2O3–SiO2–H2O lation [76].
[58].
Relative humidity/curing conditions
Calcination
Experiment showed that samples cured at higher
Calcined source material such as fly ash, slag and humidity (in sealed bags) do not improve strength.
metakaolin display a higher reactivity during geopoly- This behavior is in contrast with what expected from
merization than non-calcined material. Calcination the curing of cementitious products. Cement gains
activates material by changing their crystalline struc- strength when cured under higher humidity. This
ture into amorphous structure to store extra energy behavior is also proved by the IR absorption peaks
and increase their activity [23] and increasing com- around 1033 cm–1 corresponding to the asymmetric
pressive strength [36]. The burning or calcining tem- stretching of Si–O and Al–O bonds which were
perature of the clay affects the pozzolanic reactive affected by curing of samples in sealed bags and their
state when the calcining leads to loss of hydroxyls and wave numbers were slightly lower than for the samples
results in a collapsed and disarranged clay structure. cured without bags. Lower wave number is an indic-
The production of the active state is usually in the ative of weaker inter-tetrahedral bonding and could
range of 600–800 °C [33]. Calcination also affects the contribute to the lower strength for the samples cured
amount of Al and Si released from source material in the sealed bags. Saturated atmosphere in the bags,
(Fig. 8). Metakaolin (calcined form) persists until the results in conditions more suitable to the formation of
material is heated upto the temperature of about the slightly weaker bonds [13].
950 °C [74]. CaO content increases upon calcination.
High CaO content decreases the microstructural Age of concrete
porosity and in turn strengthen the geopolymer by
forming amorphous structure Ca–Al–Si gel during Geopolymer gains about 70% of its strength in first 3–4 h
geopolymerization [14, 18, 23, 53]. This is also sup- of the curing (Table 1) [8, 62, 77]. Strength of concrete
ported by the investigation [27] of ground granulated does not vary with the age of concrete when cured for
blast furnace slag which explains that calcium contain- 24 h, which is in contrast to well-known behavior of
ing compounds such as calcium silicates, calcium ordinary Portland cement, which undergo hydration
aluminate hydrates, and calcium-silico-aluminates are process and gains strength overtime [2].
formed during geopolymerization of fly ash, that
affects the setting and workability of the mix [18].
Higher the temperature used during the calcinations Engineering properties of geopolymers
process, shorter the time needed to obtain metakaolin
that gives the maximum compressive strength [13]. The Wallah et al. [78] performed creep and drying shrink-
geopolymers manufactured from calcined material age tests to evaluate the long-term performance and
were found to have higher early strength, while those durability of geopolymer concrete. Results indicated
formed from non-calcined materials possessed higher that geopolymer concrete undergoes low creep and
increase in strength during the later stages of curing very little drying shrinkage. Test results of Hardjito
[23]. Jaarsveld et al. [14] reported anomalous result; et al. [30] showed that the drying shrinkage strain of fly
greater strength was obtained for the geopolymers ash based geopolymer concretes were found to be
containing kaolin. insignificant. The ratio of creep factor (strain-to-elastic
The heating of kaolinite at high temperature stain) reached a value of 0.30 in approximately
(750 °C) for 24 h produces the complete hydroxyl- 6 weeks. Beyond this time, the creep factor increased
ation of the sample. The thermal pre-activation of marginally.

123
J Mater Sci (2007) 42:729–746 741

Fig. 7 Effect of water-to- 80

90 °C
solid ratio on compressive
70 75 °C

Compressive strength(MPa)
strength at different curing 45 °C
temperature 60 30 °C

50

40

30

20

10

0
0.15 0.16 0.17 0.18 0.19 0.2 0.21 0.22 0.23 0.24 0.25
Water-to-solid ratio

Fig. 8 Si and Al release in 7

Si Al
NaOH solution at different
calcination temperature 6
NaOH solution after 20 hre
Monomers release in 10%

0
0 20 40 60 80 100 120 140 160 180 200
Burning duration (min)

Hardjito et al. [79] studied the stress and strain geopolymers mortars remained stable and showed
behavior of fly ash based geopolymers and compared it negligible deterioration in the microstructure and
with the portland cement. They reported that the strength after being soaked in seawater, sodium sulfate
Young’s modulus, Poisson’s ratio, Tensile strength of solution and sulfuric acid solution [31].
fly ash based geopolymer concretes was of same Davitovits [80] performed the acid resistance test
characteristics possessed by portland cement concrete. with 5% of HCl and H2SO4 on the geopolymers and
The measured stress-strain relations of geopolymer compared it with the traditional Portland cement
concrete also fit well with equations developed origi- matrix and some other binders. The portland cement
nally for portland cement concrete. and blended cement destroyed in the acidic environ-
It was observed that the geopolymer concrete, after ment (Fig. 9). Hardjito et al. [30] performed tests on
12 to 24 weeks of sulfate exposure, showed no signif- the resistance of fly ash based geopolymer concrete
icant effect. Results indicated that changes in the and reported that there was no significant change in the
compressive strength generally fall within + 1.0 of compressive strength, the mass and the length of the
standard deviation of the mean compressive strength geopolymer specimen. Bai et al. [81] have demon-
value and therefore, do not appear to be significant. strated relation between strength and sorptivity.
The variation of length change, as a result of sulfate Strength varied in a linear manner with sorptivity. In
exposure after 24 weeks was found to be extremely order to reduce the ingress of chloride-containing or
small and generally less than 0.02%. The expansion of sulfate-containing waste into concrete, minimization of
0.5% of the original length is considered as failure of sorptivity was found to be important.
the concrete due to sulfate attack [78]. Ramlochan Cheng and Chin [63] reported the fire resistance
et al. [59] investigated that pozzolans, which were property of geopolymers. When a 10 mm thick panel of
source of additional Al2O3, were effective in reducing geopolymer is exposed to 1100 °C flame; the measured
or eliminating long-term expansion. Metakaolin based reverse-side temperature reached 240–283 °C after

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742 J Mater Sci (2007) 42:729–746

Fig. 9 Matrix break in 5% 120

acid solution HCI
H2S04
100

80

% matrix break
60

up
40

20

0
PC Slag Ca- Aluminate GP
Matrix

35 min. Authors also observed that the fire character- alter the compressive strength and specific area.
istics could be improved by increasing the KOH or the Inclusion of Pb served to strengthen the structure
alkali concentration and amount of metakaolin. It was (Strength) where as Cu did not. Pb resulted in higher
concluded that geopolymers could be fabricated for surface area than Cu. Although Pb ions influenced the
construction purpose and have great potential for structure in terms of causing increased porosity this
engineering application. effect was offset by its contribution to structural
Introduction of ultrasonication into the geopoly- strength [20].
merization system increased the compressive strength
of geopolymers and strength increased with an increase
of ultrasonication upto a certain time. However, a
prolonged ultrasonication does not increase strength of Micro-structural characterization
the formed geopolymer, due to the polycondensation
and hardening occurring simultaneously. Ultrasonica- Microstructure of the alkali activated fly ash changes
tion enhances the dissolution of Al–Si source material with the chemical composition [82]. After geopoly-
to release more Al and Si into the gel phases, thereby merization, all the main characteristic peaks of Al–Si
improving the extent of geopolymerization [24]. minerals still remained, but decreased slightly. This
Geopolymers show high microbial stability. The suggested that Al–Si mineral did not dissolve totally
surfaces of the geopolymers remained unaffected after into the gel phase. However there were no new peaks,
4 weeks in the solution of microbes. No noticeable which means that no new major crystalline phases were
change occurred to compressive strength and leach- formed [9, 17]. The baseline broadened between 20°
ability [5]. and 40° 2h was an indicative of an increased amorph-
Addition of salts affects the geopolymeric struc- icity [13]. Palomo et al. [73] studied a series of fly ash
tures. Phosphate salts are the most effective gel samples activated under different experimental condi-
solidification retarders, followed by chloride, oxalate, tions and concluded that geopolymers are a family of
and than carbonate. Carbonate was found to accelerate materials with same basic chemical composition but
Si dissolution initially and at the same time lowered the potentially different microstructures.
solubility of Ca. Oxalate and phosphate were effective IR absorption spectroscopy seems to be suitable tool
in accelerating and increasing the extent of apparent Si to characterize geopolymeric material. The main fea-
dissolution. Chloride and oxalate may also induce ture of IR spectra was the central peak between
crystallization within the geopolymers. It was concluded 1010 cm–1 and 1040 cm–1 which is attributed to the Si–
that the solubility of the calcium in the system could be O–Si or Al–O–Si asymmetric stretching mode [20]. IR
due to the fact that both anions, C2O2– 4 and HPO4
2–
spectrum is characterized by the shift of the bending
have strong affinity for calcium [25, 26]. band of the Si–O bond (1050 cm–1) from metakaolin to
Heavy metal inclusion does not influence the basic lower frequency (990 cm –1) and a decrease of the
tetrahedral bonding blocks of the structure but it 800 cm–1 band, with the formation of a new band at
influences the structure in physical manner such as to about 720 cm–1 which was reported as characteristic of

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J Mater Sci (2007) 42:729–746 743

the polymer formed [83]. IR spectra of alkaline cross-linked type structure. The analysis of Q2(1Al)
alumino-silicate had a band associated to m3 (SiO) at and Q3(1Al) intensity confirmed that an important part
997 cm–1. Other characteristics bands of the inorganic of Al was implicated in the formation of this cross
polymer were placed at 691 and 426 cm–1. Asymmetric linked structure that could connect the neighboring
stretching of Si–O of glassy silica shifts to lower structural blocks. Jimenez et al. [35, 57] reported that
frequencies, when substitution of Si by Al takes place. the MASNMR spectra of 24 h and one week were
Absorption bands at 1207 and 1172 cm–1 were associ- similar. Five components were displayed at –88.8,
ated with the partial substitution of Si by Al in the gel –93.2, –98.2, –103.5, –109.1 ppm, which corresponds to
structure [64]. the formation of tectosilicate and peak at –87 ppm,
The study of geopolymers using X-ray diffraction is correspond to the non-dissolved mullite. The 27Al
difficult because of the fact, a large part of the structure MASNMR spectroscopy showed that the main
27
is amorphous content between 20° and 40° 2h. Al signal located at + 61.4 ppm indicates that the
The degree of disorder in geopolymers can be inferred aluminum is tetrahedral coordinated and peaks at
by the way it diffracts X-ray to form a diffraction –87.3 and –91.9 indicates the presence of Si(4Al) and
pattern. In non-crystalline state, diffraction of X-ray Si(3Al) sites[20].
results in a broad diffuse halo rather than sharp
diffraction peaks [8, 28]. Peaks were of quartz, mullite
and hematite of the crystalline component of the fly Immobilization of toxic metals by geopolymer
ash. Broad peak in region 20–30° 2h arised from glassy
phase of fly ash and peaks in the region 6–10°and 16° Presently, toxic and radioactive metals are stabilized
2h arised from alumino-silicate gel. Considerable with conventional Portland cement but the cost figures
amount of zeolites were found in cement-fly ash system are not tolerable [16, 18]. Large amount of fly ash with
blend, activated by highly alkaline multi-compound a small amount of additive and activators can be
activator, around pH 14 and cured at 70 °C [58]. utilized for the solidification/stabilization of heavy
Querol et al. [67], Brougth [84] identified Phillipsite metals. About 90 % of the heavy metals get locked
[KCa(Si5Al3)O166H2O], merlinoite [K5Ca2(Al9- into the geopolymeric matrix (Fig. 10) [34].
Si23O64).24H2O], analcime [Na(Al Si2O6). H2O] and Heavy metal immobilization may occur through a
Na–P zeolite (Na6Al6Si10O32
12H2O) by XRD after combination of physical encapsulation and chemical
alkaline hydrothermal activation. Other reaction prod- bonding into the amorphous phase of the geopoly-
ucts such as calcite [(Ca(OH)2], bayerite [Al(OH)3] meric matrix. Metal cations can be incorporated into
and nosean (Na8Al6Si6O24SO4), hydroxysodalite geopolymeric network, potentially produces leach rate
(Na6(Si6Al6O24)
8H2O) ( a low silica zeolite) were far superior to those of OPC based system [43]. Very
also obtained [28]. Through XRD analysis in alkali few literature is present on the immobilization of the
activated system, phases identified were hydrotalcite metals by geopolymers. Further research is required
(Mg6Al2CO3(OH)16
4H2O), calcite (CaCO3) and on the application of the geopolymer for metal
semi-crystalline calcium silicate hydrate [85]. Bakharev immobilization.
[58] reported that zeolitic phases chabazite and Na–P1 Geopolymerization is an emerging technology in the
(gismondine) coexists with C–S–H in fly ash–NaOH field of hazardous metals immobilization. Davitovits
and fly ash–NaOH– cement system [34]. Formation of [80] reported that the geopolymeric matrix was very
zeolite and related phases in cement system is of effective in immobilizing uranium waste and nuclear
considerable interest because significant quantity of waste. The toxic metals were locked into the three
water can be immobilized in the zeolite pores, and the dimensional geopolymeric-zeolitic framework (Fig.10).
zeolites can absorb significant amount of cation [84]. Geopolymeric matrices greatly minimize the leaching
The technique of 27Al and 29Si MAS-NMR can be used of iron, cobalt, cadmium, nickel, zinc, lead, arsenic,
for the interpreting the microstructure of geopolymer radium and uranium. Metals included in each structure
synthesized from different mixture [17]. Peak at did not seem to make any difference to the crystalline
55 ppm in the aluminum spectrum indicated the part of the spectra and it was therefore assumed that
presence of a three-dimensional spectrum of alumino- the metals bound itself into the amorphous part of the
silicate polymeric units, including the presence of low matrix. Retention of the metal in the matrix was
molecular weight polymeric units such as dimmers or directly correlated with the liberation of Si and Al from
trimmers [64, 83]. Puertas et al. [85] reported that, fly ash based geopolymers [20]. NaOH was determined
presence of Q3 unit in the activated paste with to be the most effective activator while sodium silicate
waterglass indicates tetrahedra of silicates forming determined was the least effective. All matrices were

123
744 J Mater Sci (2007) 42:729–746

Fig. 10 Percentage of toxic 120

metals locked in
geopolymeric matrix
100

% of toxic metals locked

80

60

40

20

0
Hg As Fe Mn Zr Cr Co Pb Cu V Mg
Toxic metals

generally found to be highly efficient in retaining Pb (5) Hazardous waste disposal binder for the heavy
within the matrix with the order of effectiveness: Fly metal fixation especially for nuclear waste solid-
ash > Kaolinite > K-feldspar > metakaoline [19]. The ification.
environment and the coordination number of alumi- (6) Fire resistant: Geopolymer can withstand 1000 °C
num source material have an effect on the ultimate to 1200 °C without losing function.
immobilization efficiency of the geopolymeric matrix. (7) It is also known as ‘‘Green material’’ for its low
It was observed that geopolymeric matrix synthesized energy consumption and low waste gas emission
from a six coordinated aluminum source (kaolinite) during manufacture. Thermal processing of nat-
was more stable under leaching conditions than four- ural alumino-silicates at relative low temperature
coordinated aluminum source (metakaolin). From was provides suitable geopolymeric raw material,
assumed that some chemical bonding of the metals resulting in 3/5 less energy assumption than
occurred within the matrix [14]. Zeolitic phases showed Portland cement. In addition less CO2 is emitted
excellent uptake characteristics for Pb2+. In presence of [79].
NaOH, sorption efficiency was considerably reduced, (8) Fast setting: Geopolymer obtain 70% of the final
probably due to the changing speciation of Pb in compressive strength in the first 4 h of setting.
response to pH. At near neutral pH Pb is largely in (9) Geopolymers are used as construction material.
cationic form (Pb4(OH)4 [86]. There is a relative
tendency that the matrix with the best immobilization
efficiency has the smaller pore opening as well as the Conclusions
highest compressive strength, hence it was assumed
that the immobilization take along physical encapsu- Geopolymerization is an emerging technology for
lation [14, 19]. utilization of by-products like fly ash, slag, and kiln
dust and also for the immobilization of toxic metal in
the waste. It provides a mature and cost-effective
Properties and application of geopolymers solution to many problems where hazardous residues
must be treated and stored under critical environmen-
Broad properties and applications of geopolymers can tal conditions. Geopolymer based materials are envi-
be listed as follows ronmentally friendly and need only moderate energy
to produce. CO2 emission is reduced about 80%
(1) Geopolymers possess excellent mechanical compared to that of ordinary Portland cement.
strength due to high degree of Polycondensation. Any pozzolanic compound or source of silica and
(2) Long-term durability: Geopolymer concrete or alumina that is readily dissolved in the alkaline
mortars withdraw thousands of years weathering solution can be used as a source of geopolymer
attack without much function loss. precursor species and undergoes geopolymerization.
(3) Unique high temperature properties. A polymeric structure of Al–O–Si formed during
(4) Easily recycled, adjustable coefficient of thermal geopolymerization constitutes the main building block
expansion. of geopolymeric structure.

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J Mater Sci (2007) 42:729–746 745

The rate of polymer formation is influenced by many Future study in the field of the application of
parameters. Pozzolanic reactions are accelerated by geopolymer is required for its commercial uses.
curing temperature, water content, fly ash/kaolinite
ratio, alkali concentration, initial solids content, silicate
and aluminate ratio, pH and the type of activators used References
has substantial effects on the final properties of
1. Woolard CD, Petrus K, Van Der Horst M (2000) ISSN 0378-
geopolymers. Certain synthesis limits existed for the
4738-Water SA 26:531
formation of strong products. Compositions lay in the 2. Hardjito D, Wallah SE, Sumajouw DMJ, Rangan BV (2004)
range M2O/SiO2, 0.2 to 0.48; SiO2/Al2O3, 3.3 to 4.5; ACI Mater J 101:1
H2O/M2O, 10–25; and M2O/Al2O3, 0.8 to 1.6 but the 3. Baldwin G, P.E. Rushbrook, Dent CG (1982) The testing of
hazardous waste to assess their suitability for landfill
ratio changes while working with the waste. Research
disposal. Harwell report, AERE-R10737, November 1982
on geopolymers conforms that curing temperature and 4. Wiles CC (1988) Standard handbook of hazardous waste
curing time significantly influence the compressive treatment and disposal. McGraw Hills, New York, p 7.85
strength. 5. Hermann E, Kunze C, Gatzweiler R, Kiebig G, Davitovits J
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